Silicon is readily converted into silica by the action of
oxidizing agents. Silica forms only one series of salts--the
silicates--which have in many cases a complex constitution; thus there
are a large number of double silicates, which vary among themselves, not
only in the relation of base to acid (which is the essential
difference), but also in the ratio of the bases between themselves
(which varies with almost every specimen).
Silica is detected by heating the substance with a fluoride and
sulphuric acid in a platinum-crucible. On holding a rod, moistened with
a drop of water, over the escaping fumes, the white crust of silica
formed on the drop of water shows its presence. The insolubility of a
fragment of the mineral in a bead of microcosmic salt, is also a very
good test; the fragment, on prolonged heating, does not lose its angular
form.
There is no dry assay for this substance, nor volumetric method; when
the determination is required, it is carried out gravimetrically and,
generally, by the following plan.
If the sample contains oxides, sulphides, &c., in any quantity, these
are first dissolved out by treatment with acid, evaporated to dryness,
taken up with hydrochloric acid, and filtered. The dried residue is
treated in the same way as the silicates. Some silicates are completely
decomposed by such treatment; but it saves time (unless one is sure that
no undecomposable silicate is present) to treat these in the same way as
the others.
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